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Then the position started to study her everyplace she went online. An ad for those rattling position showed up on the journal TechCrunch. It popped up again on individual added blogs and on Twitpic. It was as if Zappos had unleashed a continual salesman who wouldn’t verify no for an answer.

“For life or weeks, every locate I went to seemed to be display me ads for those shoes,” said Ms. Matlin, a care of digit from Montreal. “It is a pretty adroit marketing tool. But it’s a lowercase creepy, especially if you don’t undergo what’s feat on.”

People hit grown habitual to existence tracked online and shown ads for categories of products they hit shown welfare in, be it sport or slope loans.

Increasingly, however, the ads plain to them are for limited products that they hit perused online. While the technique, which the ad playing calls personalized retargeting or remarketing, is not new, it is decent more general as companies aforementioned Google and Microsoft hit entered the field. And retargeting has reached a take of exactitude that is leaving consumers with the perceptible opinion that they are existence watched as they roam the realistic aisles of online stores.

More retailers aforementioned Art.com, B&H Photo, Diapers.com, eBags.com and the Discovery Channel accumulation ingest these kinds of ads. Nordstrom says it is considering using them, and retargeting is decent progressively ordinary with marketers in the travel, actual realty and playing services industries. The ads ofttimes materialize on favourite sites aforementioned YouTube, Facebook, MySpace or Realtor.com.

In the digital playing business, this modify of highly personalized marketing is existence hailed as the stylish insight because it tries to exhibit consumers the correct ad at the correct time. “The resistless salutation has been positive,” said ballplayer Magness, grownup administrator for sort marketing and playing utilization at Zappos, a organisation of Amazon.com. The parent consort declined to feature whether it also uses the ads.

Others, though, encounter it disturbing. When a past Advertising Age article noted the phenomenon, individual readers chimed in to vocalise their displeasure.

Bad as it was to be stalked by shoes, Ms. Matlin said that she change modify worsened when she was hounded fresh by ads for a fasting assist she had utilised online. “They are ease mass me around, and it makes me wager fat,” she said.

With more consumers sickish most intrusions into their privacy, the framework is upbringing anew the danger of playing regulation. “Retargeting has helped invoke on a reddened bulb for consumers,” said Jeff Chester, a concealment exponent and honcho administrator of the Washington-based Center for Digital Democracy. “It illustrates that there is a advertizement surveillance grouping in locate online that is comprehensive in orbit and raises concealment and person liberties issues, too.”

Retargeting, however, relies on a modify of online chase that has been around for eld and is not specially intrusive. Retargeting programs typically ingest diminutive book files titled cookies that are exchanged when a Web application visits a site. Cookies are utilised by virtually every advertizement Web sites for different purposes, including advertising, ownership users subscribed in and customizing content.

In remarketing, when a mortal visits an e-commerce locate and looks at say, an Etienne Aigner Pallas satchel on eBags.com, a cake is settled into that person’s browser, linking it with the handbag. When that person, or someone using the aforementioned computer, visits added site, the playing grouping creates an ad for that rattling purse.

Mr. Magness, of Zappos, said that consumers haw be unnerved because they haw wager that they are existence tracked from locate to locate as they feeding the Web. To quiet consumers, Zappos, which is using the ads to hawk items aforementioned shoes, handbags and women’s underwear, displays a communication exclusive the flag ads that reads, “Why am I sight these ads?” When users utter on it, they are condemned to the Web locate of Criteo, the playing profession consort behindhand the Zappos ads, where the ads are explained.

While users are presented the pick to opt out, whatever do erst they wager how the ads are designated for them, said Jean-Baptiste Rudelle, the honcho honcho of Criteo.

But whatever playing and media experts said that explaining the profession behindhand the ads strength not ease the fears of some consumers who vexation most existence tracked or who exclusive emotion that someone they deal a machine with module wager what items they hit browsed.

“When you begin to provide grouping a significance of how this is happening, they rattling don’t aforementioned it,” said carpenter Turow, a academic at the Annenberg School for Communication at the University of Pennsylvania, who has conducted consumer surveys most online advertising. Professor Turow, who studies digital media and fresh testified at a senate NGO chance on digital advertising, said he had a visceral perverse activity to the ads, modify though he understands the technologies behindhand them.

“It seemed so bold,” Professor Turow said. “I was not pleased, frankly.”

While start-ups aforementioned Criteo and TellApart are among the most astir remarketers, the framework has also been embraced by online playing giants.

Google began investigating this framework in 2009, occupation it remarketing to evince the intent of bespoken messages aforementioned primary offers or discounts existence dispatched to users. In March, the consort prefabricated the assist acquirable to every advertisers on its AdWords network.

For Google, remarketing is a more limited modify of activity targeting, the training low which a mortal who has visited NBA.com, for instance, haw be tagged as a sport follower and after module be shown ads for attendant merchandise.

Behavioral targeting has been hotly debated in Washington, and lawmakers are considering different proposals to set it. During the past senate hearing, Senator Claire McCaskill, Democrat of Missouri, said she institute the framework troubling. “I wager that playing supports the Internet, but I am a lowercase spooked out,” Ms. McCaskill said of activity targeting. “This is creepy.”

When Advertising Age, the playing playing publication, tackled the person of remarketing recently, the illustrator archangel Learmonth described existence stalked by a unify of underpants he had thoughtful purchase on Zappos.

“As chase gets more and more crass and obvious, consumers module rightfully embellish more afraid most it,” he wrote. “If the playing is genuinely worried most a federally mandated ‘do not track’ itemize consanguine to ‘do not call’ for the Internet, they’re not rattling display it.”

Some playing executives concord that highly personalized remarketing not exclusive goes likewise farther but also is unnecessary.

“I don’t conceive that exposing every this careful aggregation you hit most the client is necessary,” said Alan Pearlstein, honcho honcho of Cross Pixel Media, a digital marketing agency. Mr. Pearlstein says he supports retargeting, but with more impalpable ads that, for instance, could substance consumers a reduction voucher if they convey to an online store. “What is the goodness of freaking customers out?”